This invention relates generally to a system and method for detecting gas leakage from an enclosed space, such as fuel vapor leakage from an evaporative emission space of an automotive vehicle fuel system, especially to a system and method where a diaphragm pump positively pressurizes the space during a leak detection test.
A known on-board evaporative emission control system for an automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of a fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, the purge valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine""s combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an onboard diagnostic capability for determining if a leak is present in the evaporative emission space.
One known type of vapor leak detection system for determining integrity of an evaporative emission space performs a leak detection test by positively pressurizing the evaporative emission space using a positive displacement diaphragm pump. The diaphragm is reciprocated to create test pressure. Commonly owned U.S. Pat. No. 6,192,743, issued Feb. 27, 2001, discloses a module comprising such a pump.
It has been discovered that the output efficiency of such a pump may change due to factors such as temperature, age, friction, etc. As efficiency decreases, the length of time that the pump requires to create a specified pressure within a defined volume increases. Because a window of time that is available for a test may be limited, increases in the time required to create suitable test pressure for allowing a test to proceed may prevent the test from being completed within that window.
In view of this discovery, it would be desirable to provide measures for avoiding significant test time increases as pumping efficiency decreases.
The present invention is directed to a solution for avoiding such increases.
One general aspect of the invention relates to a leak detection system for a fuel system of an automotive vehicle that contains volatile fuel for operating the vehicle. The leak detection system comprises a diaphragm pump that is repeatedly stroked to pressurize vapor containment space of the fuel system during a leak detection test. A restriction is disposed between the pump and the space being pressurized to cause a graph plot of pressure at the pump outlet ahead of the restriction versus time to comprises a succession of peaks and valleys.
A further aspect includes a processor for determining the difference between the peaks and valleys and for adjusting the frequency at which the pump is stroked to maintain a substantially constant mass airflow into the space as efficiency of the pump changes.
Other aspects relate to leak detection methods involving the restriction and the processor.
It is believed that the inventive principles extend to a general method for self-compensating a volumetric pump for decreasing volumetric efficiency so as to maintain a desired mass gas flow into a closed test space being tested for leakage. The pump is operated to pressurize the space during a leak detection test. A restriction disposed between the pump and the space being pressurized to causes a real time pressure trace of pressure between the pump and the restriction to comprise a succession of pulses having peaks and valleys. Data from the pressure trace is used to adjust pump operation to cause the pump to maintain the desired mass gas flow as pump efficiency changes.